U.S. Pat. No. 5,612,753 to Poradish, et al., discusses a system where the images of two SLMs are projected onto a screen. The two images are the same size and are fully overlapped. In this arrangement there is a one to one correspondence between the pixels of each SLM. The Poradish patent makes reference primarily to the use of this parallel SLM technique for the improvement in light output and mitigation of color artifacts produced by the use of a field sequential color system that usually employs a color filter wheel and a single SLM.
Poradish does mention the use of parallel SLM devices for the mitigation of gray level contouring artifacts or increasing the dynamic range of the display, but the patent does not teach a method by which this may be accomplished.
When SLM based projection systems are to be used for a cinematic presentation, it is desirable for the image quality of the system to approach that of the film based projection systems commonly in use. This requires that the system has a high spatial resolution and that the system possesses a wide dynamic range. While film based systems easily produce contrast ranges of 1000:1 or greater, SLM based systems such as those based on DMDs are typically limited to contrast ranges of 500:1 or less.
In particular SLM devices have a finite black level. That is, the minimum displayed intensity in the off state for any SLM is greater than zero. In circumstances found in motion picture theatres this minimum black level is often found to be too high and the “gray” image visible when the picture is “black” is distracting and serves to emphasize that the system is different from a film based projection system.
SLM devices consist of a number of separately addressable picture elements or pixels in a x-y array. Different types of SLMs employ different methods for achieving a gray scale intensity control for each pixel. Some devices employ a continuously varying analog signal (such as reflective liquid crystal devices) while DMD devices employ pulse width modulation of pixel sized mirrors that switch between only two states, “on” and “off”.
A DMD is essentially an array of microscopic-sized mirrors on a computer chip. A post that allows the mirror to tilt supports each mirror. The chip includes electronic circuitry for moving each mirror between on and off positions using an electrostatic field. When the DMD mirror is in the on position light is reflected from the DMD onto a screen or other image plane. When the DMD is off the light is reflected to a “light dump” and does not reach the screen or image plane.
The mirrors are individually addressable so that which mirror is on and which is off at any given time determines the image. A memory cell associated with each mirror stores a bit of data that determines the on or off state of the address signal. Each mirror is addressed with a signal that indicates whether or not the mirror is to be on or off and therefore whether or not light is to be reflected to the image plane or screen.
A grayscale is obtained through a technique called binary pulse width modulation (PWM).
Under PWM, a frame interval T is divided into n time durations of
      1                  2        n            -      1        ,          ⁢      2                  2        n            -      1        ,          ⁢  …  ⁢                    ⁢          ,                          ⁢                        2                      n            -            1                                                2            n                    -          1                      ⁢                  ,each represents a bit in a n-bit binary word x={bnbn−1bn−2 . . . b1}. As a result, the value of binary word x is represented by y, the time duration when the mirror is on, and y can be calculated by:
      T    x    =      yT    =                  T                  i          =          1                n            ⁢                                    b            i                    ⁢                      2                          i              -              1                                                            2            n                    -          1                    The shortest time duration
  T            2      n        -    1  represents the least significant bit (LSB) of the binary word, and it is called the LSB time, expressed as:
      T    LSB    =            T                        2          n                -        1              .  
The LSB time is mainly limited by the mechanical switching time of a DMD chip. Typically, the LSB time is about 20 μsec. Therefore, the bit-depth, or the number of bits in a binary word representing the intensity of each pixel in the image formed by the DMD, is limited by both the LSB time and the frame rate. For a frame rate of 24 fps, the maximum bit-depth that a DMD can support is n=11.
Each single micro mirror on a DMD chip is a reflective device, and the total amount of output light for a binary word x is proportional to the “on-time” Tx if the optical switching time can be ignored. In fact, the optical switching time of a DMD is around 2 μs, which is about one-tenth of the LSB time, and can be neglected in the following analysis. If the input binary word x is simply encoded to be linear to scene brightness, then a DMD chip can be considered a linear device with respect to brightness.
A simplified cross section of a single DMD mirror array is shown in FIG. 1. Assume the incident light from a light source is L The light from this source falls on the mirror array, and when a mirror is in the on state the light is reflected by the mirror with an optical pixel efficiency of α<1. The output pixel brightness P can be calculated by:P=(αy+δ)L=αyL+Ldark 
The second item Ldark=δL represents the “dark level” of a DMD. This corresponds to the light reflected by a pixel of the array when the associated mirror is in the off state. This dark level is the combined result of light diffraction from mirror edges, reflection from the underlying substrate and scattering from the mirror surface, particularly around the dimple formed at the support post location. This combined effect is modeled by a factor δ and is typically:
  δ  ⪡      1                  2        n            -      1      The “dark level” determines the contrast ratio of a DMD device. It can be reduced by architectural improvement to the DMD pixels, but it may not be completed eliminated.
The simplest implementation of a PWM scheme makes the period of the most significant bit ½ of the total frame time. The period of the next most significant bit is ½ of this or ¼ of the total and so on.
At certain bit transitions, such as for example in an 8 bit system where the msb switches off, and the remaining bits switch on, that is the displayed code value switches from 1000000 binary to 01111111 binary, the temporal position of the associated light reflected by a pixel making this code transition changes from the first half of the frame interval to the second half. As a result temporal artifacts are produced that are found to be problematic when displaying moving images.
These artifacts can be reduced by avoiding such transitions, and by spreading the time that a bit is on during the frame more uniformly throughout the frame duration. This is obtained for example by dividing the most significant bit into say 4 separate parts, making four equal on-off transitions for the bit during the frame time, yielding the same intensity, but splitting the on time into four parts equally spaced throughout the frame duration. Additional complexity may be introduced to further smooth the transitions that occur during a gray scale ramp and to avoid the creation of low frequencies that are more noticeable as flicker in the image.
Where full-color images are required, three DMDs may be used, one for each primary color (RGB). The light from the DMDs converges so that the viewer perceives color. Another technique is to use a single DMD and a so-called “color wheel” having sections in the primary colors so that light incident on the DMD is sequentially colored RGB. The viewer's eye then integrates the sequential images into a continuous color image.
Other types of SLMs may be similarly arranged to obtain color images.